"It's just a matter of when," MacFarlane disclosed. "It's hard to do that while you have the series going on at the same time; I think that's why it took The Simpsons 20 seasons to figure out how to do it."

MacFarlane went on to say that he wants a Family Guy film to differ from 2007's The Simpsons Movie, which centred on Bart trying to stop a maniacal environmentalist from destroying Springfield.

"We do know what the Family Guy movie will be," he explained. "The Simpsons Movie, I thought, was hilarious, but the one criticism I would have is that it's a story they probably could've done on TV. There could've been an episode that had that plotline."

He continued: "That's the challenge with animation. You pretty much can do any story you want, so what is the reason for the movie? We finally hit on the answer to that question, and it will be something that would be impossible to do on TV."

MacFarlane declined to reveal any concrete details about the Family Guy film's storyline.

Alex Borstein, Seth Green, Mila Kunis and MacFarlane are among the voice cast for Family Guy, which has run for more than 200 episodes on Fox.